1. Field of the Invention
The present invention relates to speech decoding based on a packet network, and more particularly, to a method and apparatus for concealing frame loss that are capable of reducing speech quality degradation caused by packet loss in an environment in which speech signals are transferred via a packet network, and an apparatus for transmitting and receiving a speech signal using the same.
2. Description of the Related Art
Demand for speech transmission over an Internet Protocol (IP) network, such as Voice over Internet Protocol (VoIP) or Voice over Wireless Fidelity (VoWiFi), is increasing on a wide scale. In an IP network, delay caused by jitter and packet loss caused by line overload degrade speech quality.
Packet loss concealment (PLC) methods for minimizing speech quality degradation caused by packet loss in speech transmission over an IP network include a method of concealing frame loss at a transmitting stage and a method of concealing frame loss at a receiving stage.
Representative methods for concealing frame loss at a transmitting stage include forward error correction (FEC), interleaving, and retransmission. The methods for concealing frame loss at a receiving stage include insertion, interpolation, and model-based recovery.
The methods for concealing frame loss at a transmitting stage require additional information to conceal frame loss when it occurs and an additional transfer bits for transferring the additional information. However, these methods have the advantage of preventing sudden degradation of speech quality even at a high frame loss rate.
On the other hand, with the methods of concealing frame loss at a receiving stage, a transfer rate does not increase, but speech quality is suddenly degraded as the frame loss rate increases.
Extrapolation, which is a conventional method for concealing frame loss at a receiving stage, is applied to a parameter of the most recent frame recovered without loss in order to obtain a parameter for a lost frame. In a method for concealing frame loss with G.729 using extrapolation, a copy of a linear prediction coefficient of a frame recovered without loss is used for a linear prediction coefficient of a lost frame, and a reduced codebook gain of a frame recovered without loss is used as a codebook gain of a lost frame. Further, an excitation signal for a lost frame is recovered using an adaptive codebook and an adaptive codebook gain based on a pitch value for a frame decoded without loss, or using a randomly selected pulse location and sign of a fixed codebook and a fixed codebook gain. However, the conventional technique of concealing packet loss using extrapolation exhibits low performance in predicting parameters for a lost frame and has a limited ability to conceal the frame loss.
In the conventional methods for concealing frame loss using interpolation and extrapolation at a receiving stage, parameters for frames recovered without loss immediately preceding and immediately following a lost frame are linearly interpolated to recover a current lost parameter and conceal the loss, which causes a time delay until normal frames are received following the lost frame. Further, when continuous frame loss occurs, the loss increases an interval between the frames located at either side of the lost frame and received correctly without loss, which degrades recovery performance and increases the delay.
Among the conventional methods for concealing frame loss at a receiving stage, a technique for generating an excitation signal using random combination includes randomly arranging a previous excitation signal in order to generate an excitation signal having the same function as a fixed codebook for a Code-Excited Linear Prediction (CELP) CODEC. Conventional research showed that the fixed codebook, which is an excitation signal generating element for the CELP CODEC, has a random characteristic and is affected by a periodic component. The conventional method for generating an excitation signal using random combination cannot correctly generate a noise excitation signal (serving as the fixed codebook) because it considers only the random characteristic.
Meanwhile, among the conventional methods for concealing frame loss at a receiving stage, methods for adjusting the amplitude of a recovered signal include decreasing the amplitude of the recovered signal and applying an increment from a signal before loss when continuous frame loss occurs. In these methods, change in a speech signal is not properly considered in producing the recovered signal, which degrades speech quality.